In general, today's advanced material applications are subjected to environments and stresses, which benefit from unusual combinations of material properties that cannot be met by metal alloys, ceramic compacts, or polymeric materials alone. For example, in ballistic applications, a material is sought which is lightweight and thus fuel efficient, while at the same time provides great impact absorption properties to prevent injury or mechanical failure to an underlying structure designed to be hit by shrapnel or an exploding device. In aircraft or seacraft applications, materials that are strong, light-weight and at the same time corrosion resistant are also sought. To achieve these and other unusual combinations of material properties, composite materials (i.e., a multiphase material that exhibits a significant proportion of properties of its two or more constituent phases) are employed.
There are many types of composite materials. For example, particle-reinforced composite materials, fiber-reinforced composite materials, and structural composite materials or layered composite materials are generally well-known. Each type of composite material can include two or more phases wherein one phase makes up the majority of the material and is known as the matrix material and the second phase (and potentially additional phases) make(s) up a lesser extent of the composite and can be dispersed within the matrix material or layered within the matrix material to form a sandwich. The presence of the second and additional phases affects the material properties of the composite material. That is, the material properties of the composite material are dependent upon the material properties of the first phase and the second phase (and additional phases) as well as the amounts of the included phase forming the composite. Thus, the material properties of a composite can be tailored for a specific application by the selection of specific concentrations of the phases, as well as potentially, the sizes, shapes, distribution, and orientations of the included phases.
In general, a structural composite includes two or more layers of material, wherein one or more of the layers may be formed of a composite material in and of itself (e.g., a fiber-reinforced layer or particle-reinforced layer). Each layer of a structural composite provides a different function or provides a specific material property to the structural composite. For example, in ballistic applications one layer can provide toughness to blunt or plastically deform any sharp projectile, a second layer can provide impact resistance so as to absorb kinetic energy of a ballistic that hits the composite, and a third layer can provide strength so as to maintain structural integrity of the composite even after the composite material has been hit by shrapnel or a projectile. Typically, material transitions between these layers are discontinuous. That is, there is an abrupt change in material properties across an interface formed by two of the layers. It is well known that discontinuities often lead to failures in a composite material. For example, interlaminar failure can occur as a projectile's stress wave travels through a composite plate, impacting each of the interfaces between the layers.
It is also well known that nanolaminate layers may provide enhanced material properties not achievable by their constituent materials on other length scales. For example, certain bimetallic multilayer systems exhibit an anomalous jump in elastic modulus at a specific nanoscale layer thickness, a phenomenon known as the supermodulus effect. In general, to deposit nanoscale multilayers, systems such as DC magnetron sputtering or other deposition techniques that deposit material on top of a substantially flat surface have been utilized.